Ultrasonic transducers are used to measure the flow rate of a flow in a conduit. Transit time flow metering is a common ultrasonic method to measure the mean pipe flow rate. One transducer, when excited or energized by an electronic controller, emits a sound wave through the flow which is detected by another transducer. The controller alternates between the receiver and transmitter back and forth to receive signals traveling along and against the direction of flow. The transit times of the wave are then used to calculate speed of sound of the fluid and further calculate the flow rate by using the speed of sound, path length, the transit times, and conduit dimension data.
The flow velocity has a radial distribution described by its flow profile, which is a function of the Reynolds number for the flow medium, pipe roughness, and the conduit configuration. Existing one-path ultrasonic flow meters measure volumetric pipe flow Q by multiplying the conduit cross-sectional area S times the mean area flow velocity V and using an analytical or empirical profile correction factor. However, the analytical or empirical profile correction factors are often flow rate dependent and invalid if the flow profile is not symmetrical, accuracy is thus comprised.
Also, if the fluid properties are unknown, or if the conditions upstream of the transducers are complex, a severely distorted flow profile can result. Thus, multiple transducer pairs are used to more accurately extrapolate the mean pipe flow rate, and four or more transducer pairs defining four or more paths through the flow are often used. Chordal paths (paths not on a diameter or through the conduit center) are typically used in association with wetted transducers which are in contact with the conduit flow.
Each additional path through the flow requires an additional transducer pair, typically adding to the cost and complexity of the flow meter. Also, a high number of paths require a high number of ports in the flow meter body, which could cause flow profile disturbances. Flush mounting the transducer ports can reduce such disturbances, but ultrasonic signal strength is reduced. Therefore, using numerous wetted transducers can result in less accurate measurements.
In one type of system, the flow profile can be ascertained by a range gated Doppler method. Energy is scattered by particles, bubbles or any other inhomogeneities, and the energy received by the transducers has different frequency components from the incident signals, and the differences are analyzed to calculate velocities of the scatterers. See e.g. U.S. Pat. No. 4,787,252 incorporated herein by this reference. According to this patent, each clamp-on transducer pair is located on a diameter (as clamp-on transducers must be) and each acts as a transmitter and then a receiver of energy scattered back to itself. Such transducers, however, can be difficult to engineer for industrial applications.
It is also known to combine transit time and range gated Doppler techniques. See “Development of a Novel Flow Metering System Using Ultrasonic Velocity Profile Measurement” by M. Mori et al., Experiment in Fluids, 2001, Springer-Vriag (also incorporated herein by this reference). According to that paper each clamp-on transducer pair is located along a diameter path. When, however, the properties of the fluid in the conduit are not known, transducer pairs located on diameter paths can only provide limited information and therefore inaccurate results for the mean flow rate.